Illumination system of a microlothographic projection exposure apparatus

ABSTRACT

An illumination system of a microlithographic projection exposure apparatus includes a beam deflection array including a number beam deflection elements, for example mirrors. Each beam deflection element is adapted to deflect an impinging light beam by a deflection angle that is variable in response to control signals. The light beams reflected from the beam deflection elements produce spots in a system pupil surface. The number of spots illuminated in the system pupil surface during an exposure process, during which a mask is imaged on a light sensitive surface, is greater than the number of beam deflection elements. This may be accomplished with the help of a beam multiplier unit that multiplies the light beams reflected from the beam deflection elements. In another embodiment the beam deflecting elements are controlled such that the irradiance distribution produced in the system pupil surface changes between two consecutive light pulses of an exposure process.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/646,021, filed Dec. 23, 2009, which claims benefit of to EuropeanPatent Application EP 08 022 311.8, filed Dec. 23, 2008. The contents ofthese applications are hereby incorporated by reference in its entirety.

BACKGROUND

The disclosure generally relates to illumination systems ofmicrolithographic exposure apparatus that image a mask onto a lightsensitive surface. More particularly, the disclosure relates to suchillumination systems that contain an array of mirrors or other beamdeflection elements. The disclosure further relates to a method ofoperating a microlithographic projection exposure apparatus.

Microlithography (also called photolithography or simply lithography) isa technology for the fabrication of integrated circuits, liquid crystaldisplays and other microstructured devices. The process ofmicrolithography, in conjunction with the process of etching, is used topattern features in thin film stacks that have been formed on asubstrate, for example a silicon wafer. At each layer of thefabrication, the wafer is first coated with a photoresist which is amaterial that is sensitive to radiation, such as deep ultraviolet (DUV)or extreme ultraviolet (EUV) light. Next, the wafer with the photoresiston top is exposed to projection light in a projection exposureapparatus. The apparatus projects a mask containing a pattern onto thephotoresist so that the latter is only exposed at certain locationswhich are determined by the mask pattern. After the exposure thephotoresist is developed to produce an image corresponding to the maskpattern. Then an etch process transfers the pattern into the thin filmstacks on the wafer. Finally, the photoresist is removed. Repetition ofthis process with different masks results in a multi-layeredmicrostructured component.

A projection exposure apparatus typically includes an illuminationsystem for illuminating the mask, a mask stage for aligning the mask, aprojection objective and a wafer alignment stage for aligning the wafercoated with the photoresist.

In current projection exposure apparatus a distinction can be madebetween two different types of apparatus. In one type each targetportion on the wafer is irradiated by exposing the entire mask patternonto the target portion in one go. Such an apparatus is commonlyreferred to as a wafer stepper. In the other type of apparatus, which iscommonly referred to as a step-and-scan apparatus or scanner, eachtarget portion is irradiated by progressively scanning the mask patternunder the projection beam along a scan direction while synchronouslymoving the substrate parallel or anti-parallel to this direction. Theratio of the velocity of the wafer and the velocity of the mask is equalto the magnification of the projection objective, which is usuallysmaller than 1, for example 1:4.

The illumination system illuminates a field on the mask that may havethe shape of a rectangular or curved slit, for example. Ideally, theillumination system illuminates each point of the illuminated field onthe mask with projection light having a well defined irradiance andangular distribution. The term angular distribution describes how thetotal light energy of a light bundle, which converges towards aparticular point in the mask plane, is distributed among the variousdirections of the rays that constitute the light bundle.

The angular distribution of the projection light impinging on the maskis usually adapted to the kind of pattern to be projected onto thephotoresist. For example, relatively large sized features may require adifferent angular distribution than small sized features. The mostcommonly used angular distributions of projection light are referred toas conventional, annular, dipole and quadrupole illumination settings.These terms refer to the irradiance distribution in a system pupilsurface of the illumination system. With an annular illuminationsetting, for example, only an annular region is illuminated in thesystem pupil surface. Thus there is only a small range of angles presentin the angular distribution of the projection light, and thus all lightrays impinge obliquely with similar angles onto the mask.

There are a variety of different ways to modify the angular distributionof the projection light in the mask plane so as to achieve the desiredillumination setting. For achieving maximum flexibility in producingdifferent angular distribution in the mask plane, it has been proposedto use mirror arrays that illuminate the pupil surface.

In EP 1 262 836 A1 the mirror array is realized as amicro-electromechanical system (MEMS) comprising more than 1000microscopic mirrors. Each of the mirrors can be tilted about twoorthogonal tilt axes. Thus radiation incident on such a mirror devicecan be reflected into almost any desired direction of a hemisphere. Acondenser lens arranged between the mirror array and the pupil surfacetranslates the reflection angles produced by the mirrors into locationsin the pupil surface. This known illumination system makes it possibleto illuminate the pupil surface with a plurality of spots, wherein eachspot is associated with one particular microscopic mirror and is freelymovable across the pupil surface by tilting this mirror.

Other illumination systems are known from US 2006/0087634 A1, U.S. Pat.No. 7,061,582 B2 and WO 2005/026843 A2. Arrays of tiltable mirrors havealso been proposed for EUV illumination systems.

SUMMARY

In general, the quality of the image produced by the projectionobjective depends very sensitively on the irradiance distribution in thesystem pupil surface. Thus, it can be desirable to be able to producevery small changes of the irradiance distribution. This is generallypossible only if a very large number of (e.g., small) spots can beproduced with the help of the mirrors in the system pupil surface.However, the manufacture and control of arrays comprising severalthousand mirrors is generally difficult and can require substantialresources. Thus there is a need for an illumination systems whichinclude an array of mirrors or other beam deflection elements forproducing a number of spots in a system pupil surface, where this numberis large compared with the complexity of the array of beam deflectionelements.

In a first aspect, the invention features an illumination systemincluding a beam deflection array of reflective or transparent beamdeflection elements. Each beam deflection element is adapted to deflectan impinging light beam by a deflection angle that is preferablyvariable in response to control signals. The illumination system furtherincludes a system pupil surface and a beam multiplier unit which isarranged between the beam deflection array and the system pupil surfacesuch that the number of light beams in the system pupil surface isgreater than the number of light beams emerging from the beam deflectionarray.

In certain aspects, the invention is based, at least partly, on theconsideration that the number of spots illuminated in the system pupilsurface can be increased, at a given time, by multiplying the lightbeams producing the spots in the system pupil surface. Thus not one butat least two spots in the system pupil surface will move simultaneouslyif the deflection angle of a beam deflection element is varied. Sincemany target irradiance distributions in the system pupil surface havecertain symmetries, for example mirror or point symmetry, complexirradiance distributions in the system pupil surface can be produced ifthe multiplied light beams emerge from the beam multiplier unit with asymmetry that is adapted to the symmetry of the target irradiancedistributions. The symmetry of the light beam emerging from the beammultiplier unit is, in turn, determined by the layout of the beammultiplier unit and its arrangement with respect to the beam deflectionarray and other components of the illumination system.

The beam multiplier unit may include elements that are known in the artto divide a light beam into two or more light beams. For example, thebeam multiplier unit may comprise a plurality of densely arrangedwedges. If the light beams are directed such that they are centered onthe edges of the wedges, they will be divided into two partial lightbeams because different portions of the light beams are refracted atsurfaces having different orientations. In a similar embodiment, smallpyramidal structures are arranged on a support plate which have asimilar effect in two dimensions so that a light beam impinging on thecorner of the pyramidal structures will be split into four partial lightbeams. However, in these cases it may be necessary to direct theincident light beams on certain locations on the wedges, the pyramidalstructures or similar arrangements of refractive elements.

This disadvantage can be avoided, for example, if the beam multiplierunit includes a beam splitter. A beam splitter may be designed such thatthe same beam splitting properties are achieved over the entire surfaceof the beam splitter, and some types of beam splitters even splitimpinging light beams into two partial light beams irrespective of theangle of incidence.

All kinds of beam splitters known in the art may be envisaged in thiscontext. For example, the beam splitter may include a birefringentmaterial that splits light beams into an ordinary and an extraordinarylight beam having orthogonal states of polarization. For producingextraordinary light beams that leave the birefringent material at somedistance from the ordinary light beams, the birefringent material shouldbe relatively thick.

Therefore beam splitters that include a beam splitting surface may bemore preferable in this respect. The beam splitting surface may beformed by a surface at which frustrated total internal reflectionoccurs, as is the case in conventional beam splitter cubes. Beamsplitting surfaces in which interference effects occur, for exampleoptical coatings or gratings applied on a support, have the advantagethat the beam splitters are less bulky than beam splitter cubes. In someembodiments, the beam splitting surface is (e.g., at leastsubstantially) polarization independent such that it splits an incominglight beam into a reflected and a transmitted light beam irrespective ofthe state of polarization of the incident light beam. In otherembodiments the beam splitting surface is polarization dependent suchthat the intensity of the reflected and the transmitted partial lightbeams (e.g., strongly) depend on the state of polarization of theincident light beam.

In some embodiments, the ratio of the transmittance versus thereflectance of the beam splitting surface is between 100 and 0.01, e.g.,between 5 and 0.2. In many embodiments the beam splitting surface has atransmittance versus reflectance ratio of about 1, or at least between0.95 and 1.05. This ensures that the partial light beams produced by thebeam splitting surface have at least substantially the same irradiance.However, it may also be considered to deliberately depart from thisrelationship. If the transmittance versus reflectance ratio differs from1, for example lies between 7/3 and 3/7, the spots produced in thesystem pupil surface can have a different irradiance. This may be usedto adjust certain pupil related quantities such as telecentricity andpole balance.

In certain embodiments, the beam splitting surface is formed by a beamsplitting coating applied on a support. As has been mentioned above,such optical coatings exploit interference effects for splitting anincident light beam into a reflected and a transmitted light beam.Usually the beam splitting coatings comprise a plurality of thin layershaving a well defined refractive index and layer thickness. By carefullyselecting these parameters it is possible to manufacture beam splittingcoatings having well defined properties for light having a certainwavelength or range of wavelengths.

In some embodiments, the beam splitting surface is plane and extendsparallel to an optical axis of the illumination system. The optical axisof the illumination system is defined by the symmetry axis of lenses andother optical elements having rotational symmetry. With such a planebeam splitting surface extending parallel to the optical axis, andpreferably containing the optical axis, it is possible to produce mirrorsymmetries in the system pupil surface which makes it possible toproduce target irradiance distribution in the system pupil surfacehaving similar symmetries.

With such beam splitting surfaces it may be advantageous if the beamsplitting surface has a transmittance versus reflectance ratio thatvaries along the optical axis. In typical arrangements the irradianceunbalance will then increase the further a spot in the system pupilsurface is illuminated away from the optical axis.

In some embodiments, an actuator for moving the beam splitting surfaceis provided. This may be advantageous if it is desirable to producetarget irradiance distributions having different symmetry properties.

The beam splitting surface may be displaced so as to displace also theplane of symmetry in the system pupil surface. Additionally oralternatively, the actuator may be configured to rotate the beamsplitting surface. This results in a corresponding rotation of the planeof symmetry in the system pupil surface. This makes it possible toproduce one of two orthogonal planes of symmetry by rotating a singlebeam splitting surface with the help of the actuator by 90°.

In certain embodiments, the actuator is capable of completely removingthe beam splitting surface from a light propagation path. This makes itpossible to add or remove a plane of symmetry in the system pupilsurface.

In some embodiments, the beam multiplier unit includes at least twoplane beam splitting surfaces arranged at an angle which at leastsubstantially equals 90°. If suitably arranged with regard to the arrayof beam deflection elements, it can be achieved that each incident lightbeam is divided into four partial light beams passing mirrorsymmetrically through the system pupil surface.

In certain embodiments, the at least two plane beam splitting surfaces,which are arranged at an angle which at least substantially equals 90°,are arranged in planes intersecting at the optical axis of theillumination system. Thus the planes of symmetry in the system pupilsurface also intersect in the center of the system pupil surface, as itis usually desired.

The beam splitter unit can include a plane mirror which is arranged withrespect to the beam splitting surface at an angle which at leastsubstantially equals 90°. Light beams impinging first on the beamsplitting surface are split into partial light beams forming spots inthe system pupil plane that form, as a result of one of the partiallight beams being reflected by the mirror, a point-symmetricalarrangement in the system pupil surface. In contrast, light beamsimpinging first on the mirror and then on the beam splitting surfaceproduce spots in the system pupil plane that are arranged mirrorsymmetrically. Thus, in some embodiments, it possible to producemultiplied spots in the system pupil surface that arepoint-symmetrically arranged, and also spots that aremirror-symmetrically arranged. This greatly increases the range ofirradiance distributions that can be produced with the help of themultiplied light beams.

Further, the beam splitting surface and the mirror may be arranged inplanes intersecting at the optical axis of the illumination system. Thusthe point defining the reference for the point-symmetry coincides withthe optical axis, and also the symmetry plane defining themirror-symmetry contains the optical axis.

If the beam deflection array is centered with respect to the opticalaxis, the overall system symmetry can be increased which simplifies thelayout and control of the beam deflection array.

An even more symmetrical arrangement may be obtained if the optical axisdivides the beam deflection array into at least two sub-arrays, whichare arranged in an inclined manner with respect to each other. This mayrequire to direct the light beams on the beam deflection sub-arrays fromdirections which are also inclined.

If the beam multiplier unit comprises a plane mirror and a beamsplitting surface, the beam multiplier unit may be configured such thateach light beam impinges first on the plane mirror and then on the beamsplitting surface, or first on the beam splitting surface and then onthe plane mirror. Then it is ensured that each light beam is dividedinto two partial light beams forming spots in the system pupil planewhich are either point-symmetrically or mirror-symmetrically arranged.

If n planar beam splitting surfaces are arranged such that they extendin planes intersecting along a common line and forming the same angle inbetween, the arrangement of the multiplied spots in the system pupilsurface will have an n-fold symmetry. Generally, the beam multiplierunit may be configured to produce a multiplied irradiance distributionin the system pupil surface which is mirror-symmetrical,point-symmetrical, or n-fold symmetrical with respect to an originalirradiance distribution that would be produced in the system pupilsurface in the absence of the beam multiplier unit.

In certain embodiments, the illumination system includes an intermediatepupil surface which is arranged between the beam deflection array andthe beam multiplier unit. The beam multiplier unit includes an imagingsystem which establishes an imaging relation between the intermediatepupil surface and the system pupil surface. Furthermore, the beammultiplier unit is configured to produce a multiplied image of anirradiance distribution which is produced by the beam deflection arrayin a portion of the intermediate pupil surface.

Although also in this embodiment light beams are multiplied, it may besaid that this embodiment makes it possible to multiply not only lightbeams as such, but a complete irradiance distribution in a pupilsurface.

If the imaging system is a telecentric objective, the light beamsemerging from the system pupil surface will have the same angulardistribution as the light beams emerging from the intermediate pupilsurface.

The beam multiplier unit can include a mirror arranged in anotherportion of the intermediate pupil surface, wherein the beam multiplierunit is configured to produce an image of the irradiance distribution onthe mirror. Thus the beam multiplier unit multiplies the irradiancedistribution, which is produced in one portion of the intermediate pupilsurface, by forming an image of this distribution in another portion ofthe intermediate pupil surface. Then the original distribution and itsimage are imaged on the system pupil surface by the imaging system.

To this end the beam multiplier unit can include a polarizationdependent beam splitter, which is arranged within the imaging system,for example in an aperture plane of an objective forming the imagingsystem. Then light reflected from the polarization dependent beamsplitter may form on the mirror an additional irradiance distribution inthe intermediate pupil surface.

In order to be able to form an image of the additional irradiancedistribution formed on the mirror, the beam multiplier unit can includea polarization unit that changes the state of polarization of lightreflected from the mirror before it impinges again on the polarizationdependent beam splitter. As a result of the change of the state ofpolarization, this light (or at least a portion thereof) can passthrough the polarization dependent beam splitter and form an image inthe system pupil plane of the additional irradiance distribution formedon the mirror.

Transparent beam deflection elements of the beam deflection array may beformed by electro-optical or acousto-optical elements, for example. Insuch elements the refractive index can be varied by exposing a suitablematerial to electric fields for ultrasonic waves, respectively. Theseeffects can be exploited to produce index gratings that direct impinginglight into various directions.

In certain embodiments, however, the beam deflection elements aremirrors that can be tilted about at least one tilt axis. If the mirrorscan be tilted about two tilt axes, the angle formed between these tiltaxes is preferably about 90°.

In certain aspects, the invention features a method of operating aprojection exposure apparatus, including providing an illuminationsystem that includes a beam deflection array of reflective ortransparent beam deflection elements, wherein each beam deflectionelement is adapted to deflect an impinging light beam by a deflectionangle that is variable in response to control signals, and a systempupil surface in which an irradiance distribution is produced by thebeam deflection array. The method further includes illuminating a maskwith light pulses, wherein the beam deflecting elements are controlledsuch that the illuminated area associated with an irradiancedistribution produced in the system pupil surface changes between twoconsecutive light pulses of an exposure process during which a mask isimaged on a light sensitive surface; and imaging the mask on a lightsensitive surface.

The number of spots illuminated in the system pupil surface can be, at agiven instant, equal to the number of beam deflection elements. However,during an exposure process, during which a mask is imaged on a lightsensitive surface, the number of spots illuminated in the system pupilsurface can be greater than the number of beam deflection elementsbecause these spots are moved during the exposure process.

The target irradiance distribution may be divided into a plurality ofpartial irradiance distributions to which different illuminated areasare associated. The beam deflecting elements are then controlled suchthat all partial irradiance distributions are successively produced inthe system pupil surface. In other words, the system pupil surface issuccessively filled with the desired target irradiance distribution.

If the mask and the light sensitive surface are moved during theexposure process so that two points on the mask spaced apart along ascan direction are illuminated during exposure time intervals havingdifferent starting times, all partial irradiance distributions should,for an arbitrary point on the mask, be produced in the system pupilsurface during the exposure time interval associated with that point.Then also in a projection exposure apparatus of the scanner type allpoints on the mask will receive the same total irradiance distributionwhich is successively produced in the system pupil surface during theexposure time interval.

In some embodiments, the partial irradiance distributions are producedin the system pupil plane during equal time intervals. For example,there may be a fixed period at which the irradiance distribution in thesystem pupil surface is changed between two or more differentconfigurations.

The illuminated areas associated with the partial irradiancedistributions may be restricted to a segment, in particular to asemicircle or a quadrant, of the system pupil surface.

In certain embodiments, the partial irradiance distributions areinterleaved. This can have the advantage that the spots produced by thebeam deflection elements in the system pupil surface have to be moved bysmall distances only. This simplifies the control of the beam deflectionelements and reduces mechanical strains.

An illumination system of a microlithographic projection exposureapparatus can be configured to illuminate a mask with light pulses andcomprises a beam deflection array of reflective or transparent beamdeflection elements. Each beam deflection element is adapted to deflectan impinging light beam by a reflection angle that is variable inresponse to control signals. The illumination system can include asystem pupil surface in which an irradiance distribution is produced bythe beam deflection array, and a control unit, wherein the control unitis configured to control the beam deflection elements such that theilluminated area associated with an irradiance distribution produced inthe system pupil surface changes between two consecutive light pulses ofan exposure process during which a mask is imaged on a light sensitivesurface.

In some embodiments, an illumination system of a microlithographicprojection exposure apparatus includes a beam deflection array includinga number of reflective or transparent beam deflection elements. Eachbeam deflection element is adapted to deflect an impinging light beam bydeflection angles that is variable in response to control signals. Theillumination system further includes a system pupil surface, in whichlight beams reflected from the beam deflection elements illuminatespots. The number of spots illuminated in the system pupil surfaceduring an exposure process, during which a mask is imaged on a lightsensitive surface, is greater than the number of beam deflectionelements.

This may be achieved either in the time domain, i.e., by producingsuccessively different irradiance distributions in the system pupilsurface during the exposure process. Alternatively or additionally, thenumber of spots illuminated in the system pupil surface is greater thanthe number of beam deflection elements at any given instant during theexposure process. This can require the use of a beam multiplier unitthat multiplies the number of light beams emerging from the beamdeflection array.

In either case this results in an illumination system in which theeffective irradiance distribution in the system pupil surface, i.e., theirradiance distribution integrated over the exposure time interval, has,for a given number of beam deflection elements, a very high resolution.This advantage may be used either to improve the resolution of theirradiance distribution in the system pupil surface with a given numberof beam deflection elements, or to reduce the number of beam deflectionelements if a predetermined resolution of the irradiance distribution inthe system pupil surface has to be achieved.

Various aspects of the invention are summarized below.

In general, in one aspect, the invention features an illumination systemof a microlithographic projection exposure apparatus that includes abeam deflection array of reflective or transparent beam deflectionelements, wherein each beam deflection element is adapted to deflect animpinging light beam by a deflection angle; a system pupil surface; anda beam multiplier unit arranged between the beam deflection array andthe system pupil surface such that the number of light beams in thesystem pupil surface is greater than the number of light beams emergingfrom the beam deflection array.

Embodiments of the illumination system can include one or more of thefollowing features. For example, the beam multiplier unit can include abeam splitter. The beam splitter can include a beam splitting surface.The beam splitting surface can have a transmittance/reflectance ratio ofabout 1. The beam splitting surface can be formed by a beam splittingcoating applied on a support. The beam splitting surface can be a planeand can extends parallel to an optical axis of the illumination system.The beam splitting surface can have a transmittance versus reflectanceratio that varies along the optical axis. The illumination system caninclude an actuator for moving the beam splitting surface.

The actuator can be configured to rotate the beam splitting surface. Theactuator can be capable of completely removing the beam splittingsurface from a light propagation path.

In some embodiments, the beam multiplier unit includes at least twoplane beam splitting surfaces arranged at an angle which at leastsubstantially equals 90°. The at least two plane beam splitting surfacescan be arranged in planes intersecting at the optical axis of theillumination system.

In certain embodiments, the beam multiplier unit includes a plane mirrorwhich is arranged with respect to the beam splitting surface at an anglewhich at least substantially equals 90°. The beam splitting surface andthe mirror are arranged in planes intersecting at the optical axis ofthe illumination system. The beam deflection array can be centered withrespect to the optical axis. The optical axis can divide the beamdeflection array into at least two sub-arrays, which are arranged in aninclined manner with respect to each other. The beam multiplier unit canbe configured such that each light beam impinges first on the planemirror and then on the beam splitting surface, or first on the beamsplitting surface and then on the plane mirror.

The beam deflection array can produce, in the absence of the beammultiplier unit, a first irradiance distribution in the system pupilsurface, and wherein the beam multiplier unit is configured to produce asecond irradiance distribution in the system pupil surface which ismirror symmetrical, point symmetrical or n-fold symmetrical with respectto the first irradiance distribution.

In another aspect, the invention features an illumination system thatincludes an intermediate pupil surface which is arranged between a beamdeflection array and a beam multiplier unit, wherein the beam multiplierunit includes an imaging system, which establishes an imaging relationbetween the intermediate pupil surface and the system pupil surface, andwherein the beam multiplier unit is configured to produce a multipliedimage of an irradiance distribution, which is produced by the beamdeflection array in a portion of the intermediate pupil surface.

Embodiments of the illumination system can include one or more of thefollowing features and/or features of other aspects. For example, theimaging system can be a telecentric objective. The beam multiplier unitcan include a mirror arranged in another portion of the intermediatepupil surface, and wherein the beam multiplier unit is configured toproduce an image of the irradiance distribution on the mirror. The beammultiplier unit can include a polarization dependent beam splitter,which is arranged within the imaging system. The objective has anaperture plane in which the polarization dependent beam splitter isarranged. The beam multiplier unit can be configured such that lightreflected from the polarization dependent beam splitter impinges on themirror. The beam multiplier unit can include a polarization unit thatchanges the state of polarization of light reflected from the mirrorbefore it impinges again on the polarization dependent beam splitter.

In another aspect, the invention features an illumination system of anypreceding aspect, characterized in that the beam deflection elements aremirrors that can be tilted about at least one tilt axis (e.g., twoorthogonal tilt axes).

In general, in a further aspect, the invention features a method ofoperating a projection exposure apparatus, including the followingsteps: (a) providing an illumination system that includes a beamdeflection array of reflective or transparent beam deflection elements,wherein each beam deflection element is adapted to deflect an impinginglight beam by a deflection angle that is variable in response to controlsignals, and a system pupil surface in which an irradiance distributionis produced by the beam deflection array; (b) illuminating a mask withlight pulses, wherein the beam deflecting elements are controlled suchthat the illuminated area associated with an irradiance distributionproduced in the system pupil surface changes between two consecutivelight pulses of an exposure process during which a mask is imaged on alight sensitive surface; and (c) imaging the mask on a light sensitivesurface.

Implementations of the method can include one or more of the followingfeatures and/or features of other aspects. For example, a targetirradiance distribution can be divided into a plurality of partialirradiance distributions to which different illuminated areas areassociated, and wherein the beam deflecting elements are controlled suchthat all partial irradiance distributions are successively produced inthe system pupil surface. The mask and the light sensitive surface canbe moved during the exposure process so that two points on the maskspaced apart along a scan direction are illuminated during exposure timeintervals (ΔT) having different starting times, and wherein, for anarbitrary point on the mask, all partial irradiance distributions areproduced in the system pupil surface during the exposure time intervalassociated with that point.

The time intervals (P), during which the partial irradiancedistributions are produced in the system pupil plane, can all be equal.The illuminated areas associated with the partial irradiancedistributions can be restricted to a segment, in particular to asemicircle or a quadrant, of the system pupil surface. The partialirradiance distributions can be interleaved.

In general, in a further aspect, the invention features an illuminationsystem of a microlithographic projection exposure apparatus, wherein theillumination system is configured to illuminate a mask with light pulsesand includes a beam deflection array of reflective or transparent beamdeflection elements, wherein each beam deflection element is adapted todeflect an impinging light beam by a deflection angle that is variablein response to control signals, a system pupil surface in which anirradiance distribution is produced by the beam deflection array, acontrol unit, wherein the control unit is configured to control the beamdeflection elements such that the illuminated area associated with anirradiance distribution produced in the system pupil surface changesbetween two consecutive light pulses (LP_(n), LP_(n+1)) of an exposureprocess during which the mask is imaged on a light sensitive surface.

Embodiments of the illumination system can include one or more of thefollowing features and/or features of other aspects. For example, thecontrol unit can be configured to divide a target irradiancedistribution into a plurality of partial irradiance distributions towhich different illuminated areas are associated, and wherein thecontrol unit is further configured to control the beam deflectingelements such that all partial irradiance distributions are successivelyobtained in the system pupil surface.

In general, in another aspect, the invention features an illuminationsystem of a microlithographic projection exposure apparatus, thatincludes a beam deflection array including a number of reflective ortransparent beam deflection elements, wherein each beam deflectionelement is adapted to deflect an impinging light beam by a deflectionangle that is variable in response to control signals, a system pupilsurface, in which light beams reflected from the beam deflectionelements illuminate spots, wherein the number of spots illuminated inthe system pupil surface during an exposure process, during which a maskis imaged on a light sensitive surface, is greater than the number ofbeam deflection elements.

Embodiments of the illumination system can include one or more featuresof other aspects.

BRIEF DESCRIPTION OF THE DRAWINGS

Various features and advantages may be more readily understood withreference to the following detailed description taken in conjunctionwith the accompanying drawing in which:

FIG. 1 is a perspective and considerably simplified view of anembodiment of a projection exposure apparatus;

FIG. 2 is a meridional section through an illumination system containedin the projection exposure apparatus shown in FIG. 1;

FIG. 3 is a perspective view of a mirror array contained in theillumination system of FIG. 2;

FIG. 4 is a meridional section through an embodiment of a beam splittingunit which includes a plane beam splitting surface;

FIG. 5 shows an irradiance distribution in the system pupil surfaceproduced by the beam splitting unit shown in FIG. 4;

FIG. 6 is a perspective view of an embodiment of a beam splitting unitwhich includes two orthogonal plane beam splitting surfaces;

FIG. 7 is a top view on the beam splitting unit shown in FIG. 6;

FIG. 8 shows an irradiance distribution in the system pupil surfaceproduced by the beam splitting unit shown in FIGS. 6 and 7;

FIG. 9 is a top view similar to FIG. 7 of another embodiment in which abeam splitting surface can be removed from the light path;

FIG. 10 shows an irradiance distribution in the system pupil surfaceproduced by the beam splitting unit shown in FIG. 9;

FIG. 11 is a top view similar to FIG. 9 of another embodiment in which abeam splitting surface can be rotated;

FIG. 12 shows an irradiance distribution in the system pupil surfaceproduced by the beam splitting unit shown in FIG. 11 in two differentpositions of the beam splitting surface;

FIG. 13 is a perspective view of an embodiment of a beam splitting unitincluding a plane beam splitting surface and a plane mirror;

FIG. 14 is a top view similar to FIG. 7 on the beam splitting unit shownin FIG. 13;

FIG. 15 shows an irradiance distribution in the system pupil surfaceproduced by the beam splitting unit shown in FIGS. 13 and 14;

FIG. 16 is a top view similar to FIG. 14 of another embodiment in whichthe beam splitting surface and the plane mirror divide the mirror arrayinto four quadrants;

FIG. 17 is a side view of the embodiment shown in FIG. 16;

FIG. 18 is a meridional section through an embodiment of a beamsplitting unit in which an irradiance distribution formed in anintermediate pupil surface is multiplied and then imaged on the systempupil surface;

FIG. 19 shows an irradiance distribution in the intermediate pupilsurface produced by the beam splitting unit shown in FIG. 18;

FIG. 20 shows an irradiance distribution in the system pupil surfaceproduced by the beam splitting unit shown in FIG. 18;

FIG. 21 is a meridional section through an embodiment of a beamsplitting unit in which the beam splitting unit comprises birefringentelements;

FIG. 22 is a top view on the beam splitting unit shown in FIG. 21;

FIG. 23 a shows a first partial irradiance distribution, which isproduced in a system pupil surface during a first period;

FIG. 23 b shows a second partial irradiance distribution, which isproduced in a system pupil surface during a second period;

FIG. 24 shows two graphs illustrating the time dependency of theirradiance for two different points in the mask plane of projectionexposure apparatus of the wafer stepper type;

FIG. 25 shows two graphs illustrating the time dependency of theirradiance for two different points in the mask plane of projectionexposure apparatus of the scanner type;

FIGS. 26 a to 26 d show four first exemplary partial irradiancedistributions, which are successively produced in a system pupilsurface;

FIGS. 27 a and 27 b show two second exemplary partial irradiancedistributions, which are successively produced in a system pupilsurface;

FIGS. 28 a and 28 b show two third exemplary partial irradiancedistributions, which are successively produced in a system pupilsurface.

DETAILED DESCRIPTION I General Structure of Projection ExposureApparatus

FIG. 1 is a perspective and highly simplified view of a DUV projectionexposure apparatus 10 that comprises an illumination system 12 forproducing a projection light beam. The projection light beam illuminatesa field 14 on a mask 16 containing minute structures 18. In thisembodiment the illuminated field 14 has approximately the shape of aring segment. However, other, for example rectangular, shapes of theilluminated field 14 are contemplated as well.

A projection objective 20 images the structures 18 within theilluminated field 14 onto a light sensitive layer 22, for example aphotoresist, which is deposited on a substrate 24. The substrate 24,which may be formed by a silicon wafer, is arranged on a wafer stage(not shown) such that a top surface of the light sensitive layer 22 isprecisely located in an image plane of the projection objective 20. Themask 16 is positioned by means of a mask stage (not shown) in an objectplane of the projection objective 20. Since the latter has amagnification of less than 1, a minified image 14′ of the structures 18within the illuminated field 14 is projected onto the light sensitivelayer 22.

During the projection, the mask 16 and the substrate 24 move along ascan direction which coincides with the Y direction. Thus theilluminated field 14 scans over the mask 16 so that structured areaslarger than the illuminated field 14 can be continuously projected. Sucha type of projection exposure apparatus is often referred to as“step-and-scan apparatus” or simply a “scanner”. The ratio between thevelocities of the mask 16 and the substrate 24 is equal to themagnification of the projection objective 20. If the projectionobjective 20 inverts the image, the mask 16 and the substrate 24 move inopposite directions, as this is indicated in FIG. 1 by arrows A1 and A2.However, stepper tools in which the mask 16 and the substrate 24 do notmove during projection of the mask may also be used.

In the embodiment shown, the illuminated field 14 is not centered withrespect to an optical axis 26 of the projection objective 20. Such anoff-axis illuminated field 14 may be necessary with certain types ofprojection objectives 20. In other embodiments, the illuminated field 14is centered with respect to the optical axis 26.

EUV projection exposure apparatus have the same basic structure.However, because there are no optical materials that are transparent forEUV radiation, only mirrors are used as optical elements, and also themask is of the reflective type.

II General Structure of Illumination System

FIG. 2 is a more detailed meridional section through the DUVillumination system 12 shown in FIG. 1. For the sake of clarity, theillustration of FIG. 2 is considerably simplified and not to scale. Thisparticularly implies that different optical units are represented byvery few optical elements only. In reality, these units may comprisesignificantly more lenses and other optical elements.

The illumination system 12 includes a housing 28 and a light source thatis, in the embodiment shown, realized as an excimer laser 30. Theexcimer laser 30 emits projection light that has a wavelength of about193 nm. Other types of light sources and other wavelengths, for example248 nm or 157 nm, are also contemplated.

In the embodiment shown, the projection light emitted by the excimerlaser 30 enters a beam expansion unit 32 in which the light beam isexpanded without altering the geometrical optical flux. The beamexpansion unit 32 may comprise several lenses as shown in FIG. 2, or maybe realized as a mirror arrangement, for example. The projection lightemerges from the beam expansion unit 32 as a substantially collimatedbeam 34. In other embodiments, this beam may have a significantdivergence. The collimated beam 34 impinges on a plane folding mirror 36provided for reducing the overall dimensions of the illumination system12.

After reflection from the folding mirror 36, the beam 34 impinges on anarray 38 of microlenses 40. A mirror array 46 is arranged in or in thevicinity to a back focal plane of the microlenses 40. As will beexplained in more detail below, the mirror array 46 comprises aplurality of small individual mirrors M_(ij) that can be tilted,independently from each other, by two tilt axes that are preferablyaligned perpendicularly to each other. The total number of mirrorsM_(ij) may exceed 100 or even several 1000. The reflecting surfaces ofthe mirrors M_(ij) may be plane, but could also be curved, if anadditional reflective power is desired. Apart from that, the mirrorsurfaces may support diffractive structures. In this embodiment thenumber of mirrors M_(ij) is equal to the number of microlenses 40contained in the microlens array 38. Thus each microlens 40 directs aconverging light beam on one mirror M_(ij) of the mirror array 46.

The tilting movements of the individual mirrors M_(ij) are controlled bya mirror control unit 50 which is connected to an overall system control52 of the illumination system 12. Actuators that are used to set thedesired tilt angles of the mirrors M_(ij) receive control signals fromthe mirror control unit 50 such that each individual mirror M_(ij) iscapable of reflecting an impinging light ray by a reflection angle thatis variable in response to the control signal. In the embodiment shownthere is a continuous range of tilt angles at which the individualmirrors M_(ij) can be arranged. In other embodiments, the actuators areconfigured such that only a limited number of discrete tilt angles canbe set.

FIG. 3 is a perspective view of the mirror array 46 comprising, for thesake of simplicity, only 8.8=64 mirrors M_(ij). Light beams 54 aimpinging on the mirror array 46 are reflected to different directionsdepending on the tilt angles of the mirrors M_(ij). In this schematicrepresentation it is assumed that a particular mirror M₃₅ is tiltedabout two tilt axes 56 x, 56 y relative to another mirror M₇₇ so thatthe light beams 54 b, 54 b′ which are reflected by the mirrors M₃₅ andM₇₇, respectively, are reflected into different directions.

Referring again to FIG. 2, the light beams reflected from the mirrorM_(ij) impinge on a beam multiplier unit 57 in which the number of lightbeams is increased, as is indicated in FIG. 2. Various embodiments ofthe beam multiplier unit 57 will be explained in more detail below. Thelight beams then pass, in the embodiment shown, through a firstcondenser 58 which ensures that the slightly diverging light beamsimpinge, now as at least substantially parallel light beams, on anoptical integrator 72 which produces a plurality of secondary lightsources. The optical integrator 72 increases the range of angles formedbetween the light rays and an optical axis OA of the illumination system12. In other embodiments, the first condenser 58 is dispensed with sothat the light beams impinging on the optical integrator 72 have alarger divergence. In still other embodiments the beam multiplier unit57 is arranged between the first condenser 58 and the optical integrator72.

The optical integrator 72 is realized, in the embodiment shown, as afly's eye lens comprising two substrates 74, 76 that each includes twoorthogonal arrays of parallel cylindrical microlenses. Otherconfigurations of the optical integrator are envisaged as well, forexample integrators comprising an array of microlenses that haverotationally symmetrical surfaces, but rectangular boundaries. Referenceis made to WO 2005/078522 A, US 2004/0036977 A1 and US 2005/0018294 A1,in which various types of optical integrators suitable for theillumination system 12 are described.

Reference numeral 70 denotes a system pupil surface of the illuminationsystem 12 that substantially defines the angular distribution of thelight impinging on the mask 14. The system pupil surface 70 is usuallyplane or slightly curved and is arranged in or in immediate vicinity tothe optical integrator 72. As the angular light distribution in thesystem pupil surface 70 directly translates into an intensitydistribution in a subsequent field plane, the optical integrator 72substantially determines the basic geometry of the illuminated field 14on the mask 16. Since the optical integrator 72 increases the range ofangles considerably more in the X direction than in the scan directionY, the illuminated field 14 has larger dimensions along the X directionthan along the scan direction Y.

The projection light emerging from the secondary light sources producedby the optical integrator 72 enters a second condenser 78 that isrepresented in FIG. 2 by a single lens only for the sake of simplicity.The second condenser 78 ensures a Fourier relationship between thesystem pupil surface 70 and a subsequent intermediate field plane 80 inwhich a field stop 82 is arranged. The second condenser 78 superimposesthe light bundles, which are produced by the secondary light sources, inthe intermediate field plane 80, thereby achieving a very homogenousillumination of the intermediate field plane 80. The field stop 82 maycomprise a plurality of movable blades and ensures sharp edges of theilluminated field 14 on the mask 16.

A field stop objective 84 provides optical conjugation between theintermediate field plane 80 and the mask plane 86 in which the mask 16is arranged. The field stop 82 is thus sharply imaged by the field stopobjective 84 onto the mask 16.

III Beam Multiplier Unit 1. First Embodiment

FIG. 4 is an enlarged cutout from FIG. 2 showing a beam multiplier unit57 according to a first embodiment in a meridional section. As it isalso shown in FIG. 2, the beam multiplier unit 57 is arranged in thelight path between the mirror array 46 and the first condenser 58, thelatter being arranged at some distance in front of the system pupilsurface 70. In this embodiment the beam multiplier unit 57 comprises abeam splitter 88 formed by a thin support plate 90 having planar planesurfaces and a beam splitting coating 92 applied on one of thesesurfaces. The beam splitting coating 92 is formed in this embodiment bya stack of thin dielectric layers 94 having alternating indices ofrefraction. The indices of refraction and the layer thicknesses aredetermined such that the beam splitting coating 92 has, for theprojection light produced by the light source 30, a transmittance T anda reflection R with T=R≈50%. In other embodiments the T/R ratio may bedistinct from 1.

The beam splitter 88 extends in a plane which contains the optical axisOA of the illumination system 12. The optical axis OA is the axis ofrotational symmetry of the lenses and other rotationally symmetricaloptical elements of the illumination system 12.

For the sake of simplicity the light beams produced by the microlenses40 of the array 38 and directed towards the mirrors M_(ij) will berepresented in the following only by their principal rays. In FIG. 4 twosuch light beams 96, 98 are reflected from mirrors M_(ij) of the array46 and impinge on the beam splitter 88. At the beam splitting coating 92the light beams 96, 98 are split into two partial light beams 96T, 96Rand 98T, 98R, respectively. Apart from a small displacement resultingfrom refraction at the support plane 90, the transmitted partial lightbeams 96T, 98T are extensions of the incoming light beams 96 and 98,respectively. The propagation directions of the reflected partial lightbeams 96R, 98R are determined by the law of reflection, i.e., the angleof reflection equals the angle of incidence in the plane of incidence.The transmitted and reflected partial light beams 96T, 98T, 96R, 98Rthen propagate through the first condenser 58 and intersect the systempupil surface 70.

FIG. 5 illustrates the intensity distribution that is obtained under theconditions shown in FIG. 4 in the system pupil surface 70. It can beseen that the reflected partial light beams 96R, 98R produce spots inthe system pupil surface 70 which are arranged mirror-symmetrically withregard to the spots produced by the transmitted partial light beams 96T,98T, with 100 denoting the plane of symmetry in the system pupil surface70. This is a result of the planar configuration of the beam splittingcoating 92. Since the beam splitter 88 contains the optical axis, thesymmetry plane 100 in the system pupil surface 70 also contains theoptical axis OA.

The beam multiplier unit 57 thus makes it possible to produce from eachincoming light beam 96, 98 a pair of exit light beams that illuminatespots in the system pupil surface 70 being arranged mirror-symmetricallywith regard to the symmetry plane 100. By tilting a particular mirrorM_(ij) of the array 46, it is thus possible to move a pair of spots inthe system pupil surface 70 which always remain their mirror symmetrywith regard to the symmetry plane 100. For example, if the spot producedby the transmitted partial light beam 96T is moved towards the opticalaxis OA, the symmetrical spot produced by the partial light beam 96Rwill move towards the optical axis OA, too.

More generally speaking, the beam multiplier unit 57 according to thisembodiment makes it possible to produce 2N light spots in the systempupil surface 70 with only N mirrors M_(ij). This effect can beexploited to reduce the total number of mirrors M_(ij) in comparison toexisting illumination systems whilst keeping the number of spots in thesystem pupil surface 70 (referred to in the following as settingresolution) the same. Alternatively, the number of mirrors M_(ij) iskept the same as in existing systems, and the setting resolution isdoubled.

The beam multiplier unit 57 shown in FIG. 4 is ideally suited forproducing illumination settings having a mirror symmetry. Most of theusual illumination settings, namely conventional, annular, dipole andquadrupole illumination settings, have such a mirror symmetry. If thesymmetry plane 100 in the system pupil surface 70 shall be changed, thebeam splitter 88 may be coupled to an actuator 102. The actuator 102 isconfigured to rotate the beam splitter 88 around a rotational axis thatcoincides with the optical axis OA. By rotating the beam splitter 88 by90°, the symmetry plane 100 in the system pupil surface 70 will also betilted by 90°.

Another advantage of the beam multiplier unit 57 is that smaller tiltangles of the mirror M_(ij) can be used to illuminate any arbitrarylocation in the system pupil surface 70. Smaller tilt angles simplifythe construction and control of the mirrors M_(ij).

2. Second Embodiment

FIG. 6 is a perspective view of a beam multiplier unit 57 according to asecond embodiment. This embodiment differs from the embodiment shown inFIG. 4 in that the beam multiplier unit 57 comprises a second beamsplitter 89 which has the same general structure as the first beamsplitter 88, but is arranged perpendicular to the optical axis OA. Thetwo beam splitters 88, 89 therefore form a right angle in between, withthe optical axis OA running through the line where the planes, in whichthe beam splitters 88, 89 are arranged, intersect.

FIG. 7 is a top view on the beam multiplier unit 57 along the Zdirection which is assumed to be parallel to the optical axis OA. Inthis top view it can be seen that the beam splitters 88, 89 define fourquadrants, with the mirror array 46 being arranged in one of them. As aresult of this arrangement, each light beam reflected from a mirrorM_(ij) impinges twice on beam splitters, namely first on the first beamsplitter 88 and then on the second beam splitter 89, or vice versa. InFIG. 7 it is assumed that a light beam 96 reflected from a mirror M_(ij)impinges first on the first beam splitter 88 and is split into atransmitted partial light beam 96T and a reflected partial light beam96R. The transmitted partial light beam 96T impinges on the second beamsplitter 89 and is split into a reflected partial light beam 96TR and atransmitted partial light beam 96TT.

The reflected partial light beam 96R which has been produced by thefirst beam splitter 88 impinges on the second beam splitter 89 and issplit into a transmitted partial light beam 96RT and a reflected partiallight beam 96RR. As a result, the incoming light beam 96 is split intofour partial light beams 96TT, 96TR, 96RT and 96RR.

FIG. 8 illustrates the irradiance distribution obtained in the systempupil plane 70 in a representation similar to FIG. 5. The spot producedby the twice transmitted partial light beam 96TT, i.e. a light beamwhich would be present also in the absence of the beam multiplier unit57, is positioned in one quadrant of the system pupil surface 70 definedby perpendicular first and second planes of symmetry 100 and 104,respectively. The beam multiplier unit 57 produces three additionalspots, which are illuminated by the partial light beams 96TR, 96RT and96RR, in the remaining three quadrants of the system pupil surface 70.The four spots in the system pupil surface illustrated in FIG. 8 arearranged mirror-symmetrically with regard to the first and second planesof symmetry 100, 104. If the mirror M_(ij), from which the light beam 96is reflected, is tilted such that the spot produced by the partial lightbeam 96TT moves towards the optical axis OA, the other three spots willmove towards the optical axis OA as well, whilst maintaining theirmirror symmetric arrangement.

Similar to the first embodiment shown in FIG. 4, the position of thefirst and second planes of symmetry 100, 104 may be changed by changingthe position of the first and second beam splitter 88 and 89,respectively. Holders for the beam splitters 88 and/or 89 may bedisplaceable along at least one direction perpendicular to the opticalaxis OA. In other embodiments at least one of the beam splitters 88, 89is configured to be rotated about an axis coinciding with or extendingparallel to the optical axis OA. In either case the adjustability of oneof both beam splitters 88, 89 makes it possible to produce irradiancedistributions in the system pupil surface 70 having different mirrorsymmetries.

FIGS. 9 and 10 illustrate an embodiment in which the second beamsplitter comprises two portions 89 a, 89 b which can be individuallyremoved from the light propagation path with the help of actuators 106,108. A similar actuator 110 is provided for removing the first beamsplitter 88 from the light propagation path. In the configuration shownin FIG. 9 it is assumed that the actuator 106 has been operated so thatthe left portion 89 a of the second beam splitter is removed and thus nolonger exposed to any of the light beams.

FIG. 10 shows the irradiance distribution that is obtained under suchconditions in the system pupil surface 70 in a representation similar toFIG. 8. Compared to the irradiance distribution shown in FIG. 8, thespot produced by the partial light beam 96RR is missing. Additionally,the spot produced by the only once reflected partial light beam 96R hastwice the irradiance of the spots produced by the partial light beams96TT and 96TR, provided that also in this embodiment all beam splitters88, 89 having a transmittance T which equals the reflectance R.

Furthermore it has been assumed in the embodiment shown in FIG. 9 thatthe mirrors M_(ij) are controlled such that all light beams reflectedfrom the mirrors M_(ij) first impinge on the first beam splitter 88.Then the lower portion of the first beam splitter 88 shown in FIG. 7 canbe dispensed with.

FIG. 11 shows a further alternative embodiment of a beam multiplier unit57 comprising only a single beam splitter 88 which can be rotated aroundthe optical axis OA with the help of an actuator 112. If the beamsplitter 88 is rotated from the position shown with solid lines in FIG.11 to the position indicated with dotted lines such that it extendsparallel to the YZ plane, the plane of symmetry 104 in the system pupilsurface 70 changes to the plane of symmetry 100 extending parallel tothe YZ plane, as is indicated in FIG. 12. This change of the plane ofsymmetry is indicated in FIG. 12 for partial light beams 96T, 96R (forthe beam splitter 88 extending in the XZ plane) and the partial lightbeams 98T, 98R (for the rotated beam splitter 88 extending in the YZplane).

3. Third Embodiment

FIG. 13 is a perspective view on the mirror array 46 and a beammultiplier unit 57 according to a third embodiment. The beam multiplierunit 57 shown in FIG. 13 differs from the beam multiplier unit shown inFIG. 4 namely in that an additional plane mirror 114 is provided. Themirror 114 has a reflectance R close to 100% and is arranged parallel tothe optical axis OA, but perpendicular to the beam splitter 88, therebydividing the mirror array 46 in two halves of equal size.

FIG. 14 is a top view on the arrangement shown in FIG. 13 along thedirection Z direction which runs parallel to the optical axis OA. Alight beam 96 impinging first on the beam splitter 88 will be split intoa transmitted partial light beam 96T and a reflected partial light beam96R. The reflected partial light beam 96R impinges on the mirror 114 sothat it leaves the beam multiplier unit 57 diametrically opposed withrespect to the transmitted partial light beam 96T.

In the system pupil surface 70 shown in FIG. 15 the spots produced bythe transmitted and reflected partial light beams 96T, 96R are thereforearranged point-symmetrically, with the optical axis OA defining the axisof symmetry.

The same also applies to a light beam 99 which has been reflected fromthe mirror M_(ij) in the other half of the mirror array 46, butdirecting the light beam 99 again towards the beam splitter 88 so thatit is split into two partial light beams 99T, 99R. Also the spotsproduced by these partial light beams 99T, 99R are arrangedpoint-symmetrically in the system pupil surface 70.

However, if a mirror M_(ij) reflects a light beam 98 such that itimpinges first on the mirror 114, the reflected light beam 98 impingeson the beam splitter 88 such that the partial light beam 98R reflectedat the beam splitter 88 does not impinge on the mirror 114. As a result,the partial light beams 98T, 98R produce spots in the system pupil plane70 which are not arranged point-, but mirror-symmetrically, with theplane of symmetry again denoted by 100 in FIG. 15.

Thus the beam multiplier unit 57 of this embodiment makes it possible toproduce irradiance distributions in the system pupil surface 70 which donot have to be completely mirror-symmetric or point-symmetric, but mayinclude portions having point-symmetry and other portions havingmirror-symmetry. This broadens the range of irradiance distributionsthat can be produced with the help of the beam multiplier unit 57.

FIG. 16 is a top view on a beam multiplier unit 57 according to anotheralternative embodiment. Here the beam splitter 88 and the mirror 114intersect each other, which results in a cross-like arrangement ifviewed along the Z direction top. The planes, in which the beam splitter88 and the mirror 114 extend, intersect along a line which coincideswith the optical axis OA and is centered with respect to the mirrorarray 46. Thus the beam multiplier unit 57 of this embodiment dividesthe mirror array 46 not only in two halves, as it is the case in theembodiment shown in FIGS. 13 and 14, but in four quadrants 46 a, 46 b,46 c and 46 d of preferably equal size. Thus a highly symmetric layoutis achieved.

In this embodiment it may be necessary to design the mirror 114 with areduced dimension along the Z direction in comparison to the beamsplitter 88. This ensures that the transmitted partial light beams 96T,99T are not reflected again at an upper portion of the mirror 114.

FIG. 17 is a side view of the alternative embodiment shown in FIG. 16.As can be seen, the four quadrants 46 a, 46 b, 46 c and 46 d of themirror array 46 are arranged in an inclined manner with respect to eachother. Each quadrant 46 a, 46 b, 46 c and 46 d is individuallyilluminated with light beams having different offset directions, as isindicated by arrows 116, 118 in FIG. 17. Additional optics are requiredin the illumination system 12 to achieve this illumination of thequadrants 46 a, 46 b, 46 c and 46 d with light beams having differentoffset angles.

In all embodiments described above it has been assumed that thetransmittance T of the beam splitters 88 and/or 89 is equal to theirreflectance R. This ensures that the spots illuminated by the lightbeams in the system pupil surface 70 have equal irradiance, too.However, it may also be envisaged to deliberately depart from thiscondition so that the multiplied spots have different irradiances. Thismay be advantageous for example, in the embodiments shown in FIGS. 13 to17, in which light beams reflected from different mirrors M_(ij) can bedirected towards the same location in the system pupil surface 70. Ifthe T/R ratio of the beam splitters 88 is distinct from 1 in differentportions of the beam splitter 88, this provides a degree of freedom toperform fine adjustments of the irradiance distribution in the systempupil surface 70.

The embodiments shown in FIGS. 6 to 17 are particularly suited also forEUV illumination systems. A EUV beam splitter may be realized as areflective diffractive element which diffracts an incoming beam at leastalong to different directions. For example, such a diffractive elementmay be designed such that all diffraction orders other than 0, +1 and −1are suppressed.

4. Fourth Embodiment

FIG. 18 is a meridional section through a beam multiplier unit 57according to a fourth embodiment. Here the beam multiplier unit 57includes an intermediate pupil surface 120 and an objective 122 whichestablishes an imaging relation between the intermediate pupil surface120 and the subsequent system pupil surface 70. The intermediate pupilsurface 120 is either directly illuminated by the mirrors M_(ij) of thearray 46, or an additional condenser 124 indicated in broken lines isused to translate the angles of the light beams reflected from themirrors M_(ij) into locations in the intermediate pupil surface 120.

The objective 122 comprises a first positive lens 126 and a secondpositive lens 128 having a focal length f₁ and f₂, respectively. Thedistance between the first lens 126 and the second lens 128 equalsf₁+f₂, i.e. the back focal plane of the first lens 126 coincides withthe front focal plane of the second lens 128 in an aperture plane 129.The objective 122 is therefore telecentric both on the object and theimage side so that the principal rays on the object and the image siderun parallel to the optical axis OA. The intermediate pupil surface 120is arranged in a front focal plane of the first lens 126, and the systempupil plane is arranged in a back focal plane of the second lens 128. Itis to be understood that the first and second lens 126, 128 may bereplaced in other embodiments by objectives comprising two or morelenses or other optical elements.

In the aperture plane 129 a polarization dependent beam splitter 130 isarranged. The polarization dependent beam splitter 130 is configuredsuch that it has a transmittance T of nearly 100% for a first state ofpolarization and a reflectance R of nearly 100% for a second state ofpolarization which is orthogonal to the first state of polarization. Inthe embodiment shown the polarization dependent beam splitter 130 isconfigured such that p-polarized light indicated with double arrows istransmitted, whereas s-polarized light, which is indicated with a dotcentered in a circle, is reflected.

In one half of the intermediate pupil surface 120 a plane mirror 132 isarranged which may have the contour of a ring segment. In the light pathbetween the polarization dependent beam splitter 130 and the mirror 132a quarter-wave plate 134 is arranged. Between the second lens 128 andthe system pupil surface 70 there is, in the embodiment shown, anoptional retarder plate 136 which is configured to rotate thepolarization direction of linearly polarized light. This rotatingproperty may be locally varying over the surface of the retarder plate136. Examples of such retarders are described in patent applications US2002/0176166 A1 and US 2006/0055909 A1 (see FIG. 23).

The beam multiplier unit 57 functions as follows:

During operation, the mirrors M_(ij) of the array 46 are tilted suchthat the reflected light beams 96 impinge on the optional condenser lens124 and produce spots in the lower half of the intermediate pupilsurface 120. An exemplary irradiance distribution produced by the lightbeams 96 in the intermediate pupil surface 120 is shown in FIG. 19. Itis assumed that the light beams 96 are unpolarized when they passthrough intermediate pupil surface 120. If this assumption is notfulfilled, an additional depolarizer may be arranged somewhere in thebeam path in front of the polarization dependent beam splitter 130.

The light beams 96 having passed the first lens 126 converge towards theback focal point of the first lens 126 in which the polarizationdependent beam splitter 130 is arranged. Here the s-polarized lightportion of the light beams 96 is completely reflected towards the otherhalf of the first lens 126. The reflected light beams 96R propagatesthrough the quarter-wave plate 134 and impinge on the mirror 132. Sincethe mirror 132 is arranged in the front focal plane of the first lens126, an image of the irradiance distribution in the lower half of theintermediate pupil surface 120 is produced in the upper half of thesurface 120 where the mirror 132 is arranged. This image formed on themirror 132 is mirror-symmetrical with regard to the irradiancedistribution produced by the light beam 96 in the lower half of theintermediate pupil surface 120, as is illustrated also in FIG. 19.

The light beams 96R reflected from the mirror 132 take the same way backto the polarization dependent beam splitter 130, i.e. they pass thequarter-wave plate 134 and the first lens 126. Since the reflected lightbeams 96R pass the quarter-wave plate 134 twice, the state ofpolarization is converted from s-polarization to p-polarization. Due tothis conversion of the state of polarization, the polarization dependentbeam splitter 130 now transmits the reflected light beam 96R which thenpasses also through the second lens 128 and the optional retarder plate136. Thus an image of the irradiance distribution formed on the mirror132, which is itself an image of the irradiance distribution formed bythe incoming light beams 96 in the lower half of the intermediate pupilsurface 120.

Since each image is point-symmetrical with respect to the object, theirradiance distribution produced by the reflected light beams 96R in thelower half of the system pupil plane 70 is identical to the irradiancedistribution in the lower half of the intermediate pupil surface 120 asit is produced by the incoming light beams 96. This is also illustratedin FIG. 20 showing the irradiance distribution in the system pupilsurface 70 which is obtained with the exemplary irradiance distributionin the intermediate pupil surface 120 shown in FIG. 19.

The upper half of the irradiance distribution in the system pupil plane70 is formed by the p-polarized the light beams 96T that have beentransmitted by the beam splitter 130. This transmitted light beams 96Talso pass through the second lens 128 and the optional retarder plate136 and form a point-symmetrical image of the irradiance distributionproduced by the light beams 96 in the lower half of the intermediatepupil surface 120.

The beam multiplier unit 57 therefore adds a point-symmetrical image ofan irradiance distribution produced in one half of the intermediatepupil surface 120 on the other half, and then images this combinedirradiance distribution on the system pupil surface 70.

This image is magnified if the focal length f₂ of the second lens 128 isgreater than the focal length f₁ of the first lens 126.

The effect produced by the beam multiplier unit 57 is thus differentfrom the first embodiment shown in FIG. 4, because there the irradiancedistribution added by the beam multiplier unit 57 is mirror-symmetrical,whereas the beam multiplier unit 57 of the fourth embodiment shown inFIG. 18 adds a point-symmetrical irradiance distribution.

The beam multiplier unit 57 shown in FIG. 18 has also the advantage thatit is, as a kind of side effect, capable of producing (depending on theproperties of the polarization dependent beam splitter 130) fromincoming unpolarized light beams 96 either s- or p-polarized light beams96R, 96T in the system pupil surface 70. From this defined state ofpolarization it is easy to produce almost any arbitrary linearpolarization distribution with the help of the retarder plate 136.

Another advantage is that the beam multiplier unit 57 in fact multipliesan irradiance distribution produced in the intermediate pupil surface120, and not only individual light beams. Thus using the beam multiplierunit 57 of this embodiment does not necessitate any redesign of themirror array 46 and its control unit 50, as is the case with most of theembodiments described above.

For the embodiment shown in FIG. 18 a crucial element is thepolarization dependent beam splitter 130. The polarization dependentbeam splitting property can usually be achieved only for a restrictedrange of angles of incidence, for example angles between 50° and 70°.For an incoming light beam 96′, which is indicated with a dotted line inFIG. 18 and which runs close to the optical axis OA of the objective122, the polarization dependent beam splitting property of the splitter130 may not be available. Therefore this embodiment is less suited forproducing illumination settings in which a central region in the systempupil plane 70 is supposed to be illuminated.

5. Fifth Embodiment

FIG. 21 is a meridional section through a beam multiplier unit 57according to a fifth embodiment. Here the beam multiplier unit 57comprises a polarizing beam splitter which, in the embodiment shown,includes four plates 140 a, 140 b, 140 c, 140 d which are arranged in aregular pattern as is shown in the top view of FIG. 22. The plates 140 ato 140 d may be joined at their lateral faces seamlessly or, as shown inFIGS. 21 and 22, be spaced apart by a small gap.

Each of the plates 140 a to 140 d is made of a birefringent material sothat it splits up incoming light beams 96, 98 into ordinary light beams96 o, 98 o and extraordinary light beams 96 e, 98 e. The ordinary lightbeams 96 o, 98 o and the extraordinary light beams 96 e, 98 e haveorthogonal states of polarization, as is indicated in FIG. 21 with thesymbols that have been used also in FIG. 18. The distance between theemergent light beams 96 o, 96 e depends, among others, on the thicknessof the plates 140 a to 140 d along the direction defined by the opticalaxis OA. In the embodiment shown all plates 140 a to 140 d have the samethickness, but the slow birefringent axis of the birefringent materialof each plate 140 a to 140 d is oriented differently. Thus the spotsproduced by the extraordinary light beams 96 e, 98 e are arranged atequal distances from the spots produced by the ordinary light beams 96 oand 98 o, respectively, but at different angular orientations.

Additional polarization manipulating means, for example a quarter-waveplate, may be used to produce light having a uniform state ofpolarization across the entire system pupil surface 70.

IV Beam Multiplication in Time Domain

In all embodiments described above the number of spots in the systempupil surface produced by light beams during an exposure process isgreater than the number of mirrors M_(ij) of the array 46. This appliesfor any given instant during an exposure process in which the mask 16 isimaged on the light sensitive surface 22.

However, for a successful exposure process it is not necessary that anarbitrary point on the mask 16 is illuminated from all desireddirections at the same time. Instead, it is sufficient that aftercompletion of the exposure process each point on the mask 16 has beenilluminated with projection light from all desired directions. In otherwords, the multiplication of spots produced in the system pupil surface70 may also take place chronologically, i.e. in the time domain.

This is illustrated in FIGS. 23 a and 23 b which show exemplary partialirradiance distributions in the system pupil surface 70 maintainedduring a first time interval and a successive second time interval,respectively. Each partial irradiance distribution corresponds to aparticular angular distribution of light rays impinging on theilluminated field 14 of the mask 16.

1. Wafer Stepper Type

In the following it is first assumed that the projection exposureapparatus 10 is of the wafer stepper type. During the exposure processthe mask 16 and the light sensitive surface 22 thus remain stationaryfor the total exposure time T. If, for example, the time intervals,during which the partial irradiance distributions shown in FIGS. 23 aand 23 b are produced in the system pupil surface 70, have equal lengthT/2, all points on the mask 16 will be illuminated for a time intervalof length T/2 with an angular distribution corresponding to the partialirradiance distribution shown in FIG. 23 a, and for a successive timeinterval of equal length T/2 with an angular distribution correspondingto the partial irradiance distribution shown in FIG. 23 b. Another wayof describing this effect is to say that the system pupil surface 70 isnot filled in one go, but successively with the desired target totalirradiance distribution.

Since most light sources 30 of projection exposure apparatus producelight pulses, the intervals between successive light pulses may be usedto change the partial irradiance distribution in the system pupilsurface 70. With a suitable layout of the mirror control unit 50, forexample applying a control scheme as described in international patentapplication PCT/EP2008/010918 filed on Dec. 19, 2008, it is possible totilt the mirrors M_(ij) fast enough so that the partial irradiancedistribution can be changed between two successive light pulses. In awafer stepper it is, of course, likewise possible to make a longerinterruption between two successive light pulses so that there is enoughtime to change the partial irradiance distribution in the system pupilplane 70.

2. Scanner Type

In projection exposure apparatus of the scanner type, it is notsufficient to completely fill the system pupil surface 70 during theentire exposure process, but during the (shorter) exposure timeintervals during which each point on the mask is illuminated withprojection light. These exposure time intervals are equal for all pointson the mask, but have different starting (and consequently alsofinishing) times if points on the mask are spaced apart along the scandirection. For that reason the sequence of partial irradiancedistributions produced in the system pupil surface 70 has to be repeateduntil the entire exposure process is terminated.

FIG. 24 shows two graphs in which the time dependency of irradiancesI₁(t) and I₂(t) on two points on the mask 16, which are spaced apartalong the scan direction, are represented. In the upper graph it isassumed that the first point will move into the illuminated field 14 ata time t₀ during the exposure process. I will leave the illuminatedfield 14 at a later time t₀+ΔT, with ΔT being the exposure time intervalwhich is equal for all points on the mask 16 and denotes the time apoint is illuminated during the exposure process. For the sake ofsimplicity it is further assumed that the partial irradiancedistribution produced in the system pupil 70 will be changed to theconfiguration shown in FIG. 23 a at time t₀. This partial irradiancedistribution is maintained for a time ΔT/2, as is indicated by smallrepresentations of the partial irradiance distributions between theupper and the lower graphs in FIG. 24.

After the first half of the exposure time interval ΔT the partialirradiance distribution is changed between two consecutive light pulsesLP_(n) and LP_(n+1). During the second half of the exposure timeinterval ΔT, the partial irradiance distribution shown in FIG. 23 b isproduced in the system pupil surface 70. When the exposure time intervalis finished, the first point has received an equal number of lightpulses with the partial irradiance distribution shown in FIG. 23 a andwith the partial irradiance distribution shown in FIG. 23 b. Theeffective total irradiance distribution in the system pupil plane 70 isillustrated on top of the upper graph at time t₀+ΔT.

The lower graph of FIG. 24 illustrates the same process for the secondpoint which enters the illuminated field 14 later than the first pointat a time t₁>t₀. During the exposure time interval ΔT associated withthe second point the irradiance distribution in the system pupil surface70 changes twice, namely a first time between the light pulses LP_(n)and LP_(n+1) and a second time between the light pulses LP_(n+15) andLP_(n+15+1). However, the number of light pulses impinging on the secondpoint during the time intervals, in which the partial irradiancedistribution shown in FIG. 23 a is produced in the system pupil surface70, and time intervals, in which the partial irradiance distributionshown in FIG. 23 b is produced in the system pupil surface 70, are againequal. Thus also the second point will, after the end of its exposuretime interval, be illuminated by light that is associated with the sameeffective total irradiance distribution in the system pupil surface 70as the first point.

Some illumination systems are designed such that points on the mask areilluminated with a reduced irradiance at the beginning and the end ofthe exposure time intervals Δt associated with this point. This may beaccomplished, for example, by using a field stop 82 having a pluralityof movable blades that have a transmittance gradient along theirlongitudinal direction, as is described in US 2006/0244941 A1.

FIG. 25 shows two graphs similar to the graphs shown in FIG. 24, but forthe assumption of light pulses having an increasing and a decreasingirradiance at the beginning and the end of each exposure time intervalΔT. In this case the period P, at which the irradiance distribution inthe system pupil surface 70 is changed, has to be smaller than theexposure time interval ΔT, and the incline and the decline of theirradiance of the light pulses at the beginning and the end of theexposure time interval should be symmetrical.

As can be seen best in the lower part of FIG. 25 illustrating the timedependency of the irradiance on the second point, the number of lightpulses associated with a particular partial irradiance distribution inthe system pupil surface 70 are different. However, if not the number,but also the irradiance of the light pulses are taken into account, itcan be seen that the total irradiance impinging on a point on the maskduring the exposure time interval ΔT is the same for each irradiancedistribution in the system pupil surface 70.

3. Other Examples of Partial Irradiance Distributions

As a matter of course, this concept is not restricted to only twopartial irradiance distributions that are successively produced in thesystem pupil surface 70.

FIGS. 26 a to 26 d illustrate four different partial irradiancedistributions P1 to P4 that are produced successively during theexposure process in the system pupil surface 70. During each period Pone of the poles P1 to P4 is produced in the system pupil surface 70.After four periods P each point on the mask 16 has been effectivelyilluminated with light from directions that are associated with the fourpoles P1 to P4, as is illustrated in FIG. 26 d. The exposure timeinterval ΔT has to be equal to or greater than 4P.

FIGS. 27 a and 27 b show other partial irradiance distributions that aresuccessively produced in the pupil plane 70. In this embodiment thetotal areas illuminated during each period significantly differ. If thecentral pole P0 is supposed to receive the same irradiance as the fourouter poles P1, P2, P3 and P4 produced during the other period, some ofthe mirrors M_(ij) in the array 46 have to be brought into an off-statein which they do not direct any light towards the system pupil surface70.

FIGS. 28 a and 28 b show two partial irradiance distributions that areproduced successively in the system pupil surface 70 according to stillanother embodiment. Here the partial irradiance distributions are notrestricted to certain segments of the system pupil surface, but arearranged in an interleaved manner. For the sake of simplicity the spotsproduced by each mirror M_(ij) in the system pupil surface 70 areassumed to be squares. These squares are arranged in both partialirradiance distributions in a chessboard like manner, but will offset byone square. The combination of the two partial irradiance distributionsresults in a conventional illumination setting with a uniformlyilluminated circular area. A high edge resolution is achieved althoughthe number of mirrors M_(ij) is small. This interleaved configuration ofpartial irradiance distributions has also the advantage that the mirrorsM_(ij) have only to be slightly readjusted when the irradiancedistribution is changed. This simplifies the control of the mirrorsM_(ij) and reduces the mechanical strain on the mirrors and on theactuators used to adjust the tilting angles.

Other embodiments are in the following claims.

1. A method, comprising: providing an illumination system of aprojection exposure apparatus, the illumination system comprising anarray of beam deflection elements, each beam deflection element beingreflective or transparent, each beam deflection element being adapted todeflect an impinging light beam by a deflection angle that is variable,and the illumination system having a pupil surface in which anirradiance distribution is produced by the array; and using theillumination system to illuminate a mask with light pulses, wherein thebeam deflecting elements are controlled so that an illuminated areaassociated with an irradiance distribution produced in the pupil surfaceof the illumination system changes between two consecutive light pulsesthat illuminate the mask.
 2. The method of claim 1, further comprisingimaging the mask onto a light sensitive surface.
 3. The method of claim1, wherein a target irradiance distribution in the pupil surface of theillumination system is divided into a plurality of partial irradiancedistributions in the pupil surface of the illumination system with whichdifferent illuminated areas are associated, and wherein the beamdeflecting elements are controlled so that all partial irradiancedistributions in the pupil surface of the illumination system aresuccessively produced.
 4. The method of claim 3, further comprisingmoving the mask and the light sensitive surface as the mask isilluminated with the light pulses so that two points on the mask spacedapart along a scan direction are illuminated during time intervalshaving different starting times, wherein, for an arbitrary point on themask, all partial irradiance distributions are produced in the pupilsurface of the illumination system during a time interval associatedwith that point.
 5. The method of claim 3, wherein time intervals,during which the partial irradiance distributions are produced in thepupil plane of the illumination system, are all equal.
 6. The method ofclaim 3, wherein the illuminated areas associated with the partialirradiance distributions are restricted to a segment of the pupilsurface.
 7. The method of claim 6, wherein the segment is a semicircleor a quadrant.
 8. The method of claim 3, wherein the partial irradiancedistributions are interleaved.
 9. A system, comprising: an illuminationsystem, comprising: an array of beam deflection elements, each beamdeflection element being reflective or transparent, each beam deflectionelement being adapted to deflect an impinging light beam by a deflectionangle that is variable in response to control signals, and theillumination system having a pupil surface in which an irradiancedistribution is produced by the array; and a control unit configured tocontrol the beam deflection elements so that an illuminated areaassociated with an irradiance distribution produced in the pupil surfaceof the illumination system changes between two consecutive light pulsesof an exposure process during which a mask is imaged on a lightsensitive surface, wherein the illumination system is configured to beused in a projection exposure apparatus.
 10. The illumination system ofclaim 9, wherein the control unit is configured to divide a targetirradiance distribution into a plurality of partial irradiancedistributions to which different illuminated areas are associated, andwherein the control unit is configured to control the beam deflectingelements so that all partial irradiance distributions are successivelyobtained in the pupil surface.
 11. An apparatus, comprising: the systemof claim 9; and a projection objective, wherein the apparatus is aprojection exposure apparatus.